Shadow sprite rotation regardless of the light source

Hello. Help make a shadow that appears regardless of the sprite rotation relative to the light source. The problem is that when the light source and the sprite to be in line for the Z-coordinate - the shadow of flat. I hope that it can be implemented shader. Below is a shader code and a screenshot. Thanks

 Shader "Sprites/Diffuse-Shadow"
 {
     Properties
     {
         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
         _Color("Tint", Color) = (1,1,1,1)
         _BumpMap("Normalmap", 2D) = "bump" {}
         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
         _Cutoff("Alpha Cutoff", Range(0,1)) = 0.5
     }
 
         SubShader
     {
         Tags
     {
         "Queue" = "Transparent"
         "IgnoreProjector" = "True"
         "RenderType" = "TransparentCutOut"
         "PreviewType" = "Plane"
         "CanUseSpriteAtlas" = "True"
     }
 
         Cull Off
         Lighting On
         ZWrite Off
 
         CGPROGRAM
 #pragma surface surf Lambert alpha vertex:vert addshadow nofog keepalpha alphatest:_Cutoff 
 #pragma multi_compile _ PIXELSNAP_ON
 
         sampler2D _MainTex;
         sampler2D _BumpMap;
     fixed4 _Color;
 
     struct Input
     {
         float2 uv_MainTex;
         float2 uv_BumpMap;
         fixed4 color;
     };
 
     void vert(inout appdata_full v, out Input o)
     {
 #if defined(PIXELSNAP_ON)
         v.vertex = UnityPixelSnap(v.vertex);
 #endif
         v.normal = float3(0, 0, -1);
         v.tangent = float4(1, 0, 0, 1);
         UNITY_INITIALIZE_OUTPUT(Input, o);
         o.color = v.color * _Color;
     }
 
     void surf(Input IN, inout SurfaceOutput o)
     {
         fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * IN.color;
         o.Albedo = c.rgb * c.a;
         o.Alpha = c.a;
         o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
     }
     ENDCG
     }
 
         Fallback "Transparent/VertexLit"
 }
 

Shader code